Rhodium plating



Patented Dec. 4, 1951 T- D TAIES 'OFFiICE J :f f -jlzgsm364l d v RIl ODIU M PLATIN i rank Herbert Reid, Hounslow, England, assignor to The International Nickel Company Inc.,New I York, N.'Y., a corporation. -of Delmyare No Drawing. Application August 18,1948, Serial No. 44,978. In Great Brita-in August 19, 1947 l I This invention relates to the electrodepos i tion of rhodium .onto other. metals and to solutions for use therein.

The electrodeposition of, rhodium onto other metals for decorativecor tarnish-resisting purposes is commonly eiiectedwithstheuse of electrolytes of the kindicontaining as the essential constituent rhodium sulphate .or rhodiumphosphate or a mixture ofi-thes'etwo'comp'ounds together with varying proportions .of. free sulphuric acid or free phosphoric .acid or a mixture thereof. The free acid is" necessary to prevent hydrolysis of the rhodium compounds in the electrolyte with consequentprecipitation of rhodium hydroxide.

I have found that electrolytes of this kind,

:When operated under normal plating conditions,

tend to furnish rhodium deposits which become milky in appearance as the thickness increases. The exact thickness of the deposit at which the desired metallic-brilliance'is lost varies with-the composition of the electrolyteand the operating conditions. Thus, *depositsfrmn phosphate electrolytes may become milky whenttlieyxare only 2.5 millionths of an inch thick whereas deposits from pho phate-sulphate electrolytes may remain bright until-they are as thick as millionths of an inch. .Deposits from sulphate electrolytes generally remainlbright until they" are about 5 to '7 millionths of an inch thick. Now the minimum thickness generally regarded as satis factory in industrialpractice is 5 millionths of an I have discovered that if a critical amount of phenol (hydroxybenzene) is addedito the electrolyte, the thickness oi. the rhodiumdeposits is greatly increased before their characteristic brilliance is lost orthey become milky in appearance.

The. phenol concentrationiliesi within the range 0.07 .to 0.5 gram perliter. .At concentrations' be low the lower figure no significant'improvement is vobtainedand at concentrations above the upper figure the deposits tend to be dark.

It is to "be understood; that bright deposits greater than 5-millionths of an inch thick and therefore utilisable for some purposes can-be obtained with some electrolytes, particularlythe phosphate-sulphate electrolytes, under "suitable operating conditions, but even then the use of phenol is advantageous because the deposits should :as a general rule be thicker than the minimum and the risk of inilkiness appearing :be-.

fore the desiredthickness is reached can be eliminated. It is also to be iinderstoodthat even with the use of phenol the deposits cease to be bright at thicknesses which vary with the composition of the electrolyte and the operatingiconditions.

In processe'soi the-kind in question as carried on hitherto, it is usual to employ a rhodium concentration of :2'grams per. liter of electrolyte, .a current density of from 2 to a ampere's per square dec'imeter-and an operating temperature of from room temperature to about "60 C. but more usual 15; from '35 to C. Incarrying out thepresent invention none of these figures is criticaLIbutit is preferred to depart from .the prior art practice only to the extent of adding the critical amount of phenol. Nevertheless, if desired the rhodium concentration may 'vary' between, say, about 0.5 and about 5 grams per liter. The free acid must always be enough to prevent hydrolysis of rhodium compounds, as those skilled in the art will understand. Increase in the free acid content above, .say, .20 milliliters of spulphuric acid per liter decreases the current efilciency and so is time-consuming and uneconomic. The current density must, of course, be high enough to ensure that the metal on which the deposit is being made will not be chem cally attacked, but maybe-less'than 2 amps/dmP. High cathodic current densities, "say above about 5 amps/dmF, tend to cause deterioration o f'ithe electrolyte, so I prefer to avoid their use.

I have found that if the phenolooncentra tion exceeds 49.3 gram per liter the thickness of the deposit before slight milk ness appears de creases when the'lower temperatures are used. To obtain the'best results at the higher concentrations, therefore, the higher temperatures should be used, e. g.- atlea'st 40 C. with a concentration as high as 0.4 gram of phenol per liter. The current efiiciency falls ofi rapidly as the concentration-of phenol increases, the reduction being more marked at room temperature than at higher temperatures, so the high temperatures are desirable for this reason. However, at the higher temperatures the electrolyte tends to deteriorate if the cathodic current densityv is high. For instance one electrolyte containing 0.2 gram of phenol per liter which had been furnishing bright deposits up to millionths of an inch thick at room temperature and 40 C. using a current density of 4 amps/din. was subsequently electrolysed at a temperature of 60 C. with'a current density of 10 amps/din? for 5 minutesand then gave deposits which were slightly milky when theywere only 25 millionth's of an inch-thick. On being again electrolysed at 40 C.

i with a ourrentdensity of *4 amps/elm? it failed To avoid this tendency it is desirable to maintain the current density in the normal range-of'from 2 to 4 amps/dmfi.

EXAMPLE! K An electrolyte of the phosphate-sulphate type was prepared by reducing five grams of insoluble rhodium chloride with hydrogen at a low temperature, extracting the residue of finely divided rhodium by heating with concentrated sulphuric acid, cooling, diluting, filtering and precipitating rhodium hydroxide 'from the cooled filtrate by neutralization with dilute ammonium hydroxide solution. The hydroxide was washed'thoroughly, transferred to a beaker and stirred into a thin cream with water. Solution was readily effected on adding 5 mls. of 85% phosphoric acid and warming gently. The resulting solution was poured into a mixture of 17 mls. of concentrated sulphuric acid and 850 mls. of distilled water to produce an electrolyte containing per liter 2 grams of rhodium, 20 mls. of sulphuric acid and 5 mls. of phosphoric acid.

The electrolyte prepared as described was then used to deposit rhodium onto cathodes of polished nickel, and the results shown by Table I were obtained.

An electrolyte of the sulphate type was prepared by diluting with water 25 mls. of a sulphate solution prepared as described in Example I to give a final composition as follows:

Rhodium (as sulphate) grams/liter 2 Free sulphuric acid mls./liter 10 This electrolyte was also used for depositing rhodium onto nickel and the results shown in Table II were obtained.

Table II Current Thick- Phenol Density Temp. Time ness of Appear No. concn. (ampsll Q (mins Rh (hm ance of (gmSJl) am!) X104) deposit nil 2 20 6 l0 Milky. nil 2 40 1. 5 10 D0. nil 2 1. 5 6 Do. nil 4 20 4. 2 14 D0. nil 4 4O 1. 75 7 Do. nil 4 60 2. 1 14 Do.

2 4 20 60 131 Bright 0.2 4 60 10 52 D0. 0. 2 4 60 5 31 Do. 0. 2 4 60 6 28 Do.

The concentrations of phenol quoted above are illustrative and may be varied under appropriate operating conditions and still give satisfactory plating. The efiect of variation in the phenol content is shown by the following example.

EXAMPLEIII' A sulphate electrolyte containing per liter 2 grams of rhodium and 10 mls. of sulphuric acid gave the results shown in Table III when elec- T trolysed at a current density of 4 ampsJdmF.

, 7 Table III Thick- Phenol Appear- No. concu. 63 1 fi fi agg ance of (gms./l.) V X10. deposit 0. 1 60 5 27 Bright. 0. 3 .60 12 79 D0. 0. 4 '60 21 118 D0. 9. 5 60 7 101 Do.

EXAMPLE IV A phosphate electrolyte was made by dissolving rhodium hydroxide in phosphoric acid and diluting the phosphate solution to give a concentration, per liter, of 2 grams of rhodium and 20 mls. of phosphoric acid. On electrolysing this solution with a current density of 4 amps./dm. the results shown in Table IV were obtained.

Table IV Current Thick- No {31:31:11 Density T emp. Time llzlfisspf 252323;. (gms./l.) 3 3 5 P 593 deposit 19 nil 4 20 2 2. 5 Slightly milky. 20- 0. 1 4 20 v 10 15 Bright.

. phosphate and rhodium sulfate in an amount suiiicient to provide said electrolyte with about 0.5 to about 5 grams per liter of rhodium, and free acid from the group consisting of phosphoric acid and sulfuric acid in an amount sufficient to prevent hydrolysis of the rhodium compound but not exceeding about 20 milliliters per liter of electrolyte; the improvement which comprises incorporating in said electrolyte about 0.07 to about 0.5 gram per liter of phenol whereby thick, light-colored and bright rhodium electro-deposits are obtained atthicknesses of at least about 10 millionths of an inch on metal surfaces by employing the aforesaid phenol -containingacid electrolyte. I I

2. In the process for electro-depositing thick plates of bright rhodium on metal using aqueous, acid electrolyte containing at least one rhodium compound from the group consisting of rhodium phosphate and rhodium sulfate: in an amount suflicient to provide said electrolyte with about 2 grams per liter of rhodium, and free acid from the group consisting of phosphoric acid and sulfuric acid in an amount sufficient to prevent hydrolysis of the rhodium compound but not exceeding about 20 milliliters per-liter of electrolyte; the improvement which comprises incorporating in said electrolyte about 0.07 to about 0.5 gram per liter of phenol whereby thick, lightcolored and bright rhodium electro-deposits are obtained at thicknesses of at least about millionths of an inch on metal surfaces by employing the aforesaid phenol-containing acid electrolyte.

3. A process for the production of thick, bright electro-deposits of rhodium on metal which comprises establishing a rhodium-plating electrolyte containing about 0.5 to about 5 grams per liter of rhodium provided by at least one rhodium compound from the group consisting of rhodium phosphate and rhodium sulfate, acid from the group consisting of phosphoric acid and sulfuric acid in an amount sufficient to prevent hydrolysis of the rhodium but not exceeding more than about 20 milliliters of acid per liter of electrolyte, about 0.07 to about 0.5 gram per liter of phenol; and electrolytically depositing rhodium from said electrolyte onto said metal at a current density between about 2 and about 4 amperes per square decimeter and at a temperature between about room temperature and about 60 C. to produce thick, light-colored and bright rhodium electrodeposits at thicknesses of at least about 10 millionths of an inch on said metal immersed as cathode in said electrolyte.

4. An electrolyte for use in obtaining thick electro-deposits of bright rhodium on metal which comprises an aqueous, acid bath containing about 0.5 to about 5 grams per liter of rhodium provided by at least one rhodium compound from the group consisting of rhodium phosphate and rhodium sulfate, acid from the group consisting of phosphoric acid and sulfuric acid in an amount sufficient to prevent hydrolysis of the rhodium but not exceeding about 20 milliliters of acid per liter of electrolyte, and about 0.07 to about 0.5 gram per liter of phenol whereby thick, light-colored and bright rhodium electro-deposits at thicknesses of at least about 10 millionths of an inch can be obtained on metal surfaces when employing the aforesaid phenol-containing, aqueous, acid electrolyte in electro-depositing rhodium onto metal surfaces.

5. An electrolyte for use in obtaining thick electro-deposits of bright rhodium on metal which comprises an aqueous, acid bath containing about 2 grams per liter of rhodium provided by at least one rhodium compound from the group consisting of rhodium phosphate and rhodium sulfate, acid from the group consisting of phosphoric acid and sulfuric acid in an amount sufficient to prevent hydrolysis of the rhodium but not exceeding about 20 milliliters of acid per liter of electrolyte, and about 0.07 to about 0.5 gram per liter of phenol whereby thick, light-colored and bright rhodium electro-deposits at thicknesses of at least about 10 millionths of an inch can be obtained on metal surfaces when employing the aforesaid phenol-containing, aqueous, acid electrolyte in electro-depositing rhodium onto metal surfaces.

FRANK HERBERT REID.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,895,675 Mathers Jan. 31, 1933 1,981,820 Zimmerman et a1. Nov. 20, 1934 2,078,014 Oplinger et a1. Apr. 20, 1937 2,119,304 Viers et a1 May 31, 1938 2,250,556 Stareck July 29, 1941 FOREIGN PATENTS Number Country Date 446,393 Great Britain Apr. 29, 1936 OTHER REFERENCES Transactions of the Electrochemical Society, vol. (1941), pp. 494 to 497. 

1. IN THE PROCESS FOR ELECTRO-DEPOSITING THICK PLATES OF BRIGHT RHODIUM ON METAL USING, AQUEOUS, ACID ELECTAROLYTE CONTAINING AT LEAST ONE RHODIUM COMPOUND FROM THE GROUP CONSISTING OF RHODIUM PHOSPHATE AND RHODIUM SULFATE IN AN AMOUNT SUFFICIENT TO PROVIDE SAID ELECTROLYTE WITH ABOUT 0.5 TO ABOUT 5 GRAMS PER LITER OF RHODIUM, AND FREE ACID FROM THE GROUP CONSISTING OF PHOSPHORIC ACID AND SULFURIC ACID IN AN AMOUNT SUFFICIENT TO PREVENT HYDROLYSIS OF THE RHODIUM COMPOUND BUT NOT EXCEEDING ABOUT 20 MILLILITERS PER LITER OF ELECTROLYTE; THE IMPROVEMENT WHICH COMPRISES INCORPORATING IN SAID ELECTROLYTE ABOUT 0.07 TO ABOUT 0.5 GRAM PER LITER OF PHENOL WHEREBY THICK, LIGHT-COLORED AND BRIGHT RHODIUM ELECTRO-DEPOSITS ARE OBTAINED AT THICKNESSES OF AT LEAST ABOUT 10 MILLIONTHS OF AN INCH ON METAL SURFACES BY EMPLOYING THE AFORESAID PHENOL-CONTAINING ACID ELECTROLYTE. 